Joining the Two Domains of a Group I Ribozyme to Form the Catalytic Core

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Abstract
Self-splicing group I introns, like other large catalytic RNAs, contain structural domains. Although the crystal structure of one of these domains has been determined by x-ray analysis, its connection to the other major domain that contains the guanosine-binding site has not been known. Site-directed mutagenesis and kinetic analysis of RNA splicing were used to identify a base triple in the conserved core of both a cyanobacterial ( Anabaena ) and a eukaryotic ( Tetrahymena ) group I intron. This long-range interaction connects a sequence adjacent to the guanosine-binding site with the domain implicated in coordinating the 5′ splice site helix, and it thereby contributes to formation of the active site. The resulting five-strand junction, in which a short helix forms base triples with three separate strands in the Tetrahymena intron, reveals exceptionally dense packing of RNA.